Preparation and in Vitro Digestive Analysis of Casein-Derived Peptide-Zinc Chelates

In this study, casein peptides were obtained by alcalase hydrolyzation and Lactobacilus fermentation to prepare the casein peptide-zinc chelates, which are a kind of efficient and well-absorbed zinc supplement. The structure of casein peptide-zinc chelates was characterized by spectroscopic instruments. The digestive stability and safety were also assayed using in vitro digestive model and Caco-2 cell experiment. The results indicated that the optimal conditions for preparing casein peptides were as follows: Volume of alcalase was 0.3% (w/v), pH of reaction system was 9.0, Lactobacilus fermentation time was 12 h. Under this condition, the peptide content was 142.39±0.95 mg/g, and the zinc chelation rate was 31.41%±0.97%. The chelation with zinc destroyed the structure of casein peptides leading to a loose surface structure. Spectroscopic analysis showed that Zn²⁺ can bind to the reactive groups on casein peptides, with the chelating sites being carboxyl oxygen, hydroxyl oxygen and amino groups. The results of in vitro digestion showed that casein peptide-zinc chelates possessed better solubility than that of zinc sulfate. After gastrointestinal digestion, the DPPH and ABTS⁺ scavenging ability of casein peptide-zinc chelates increased by 26.19%±3.30% and 71.96%±7.06%, respectively. However, the ferric ion reducing power of the casein peptide-zinc chelates decreased by 36.26%±2.80%. At the same time, the β-turn and random coil content of the casein peptide-zinc chelate decreases during digestion, the β-sheet structure increases and Zn²⁺ play a role in maintaining the peptide structure. Furthermore, the cytotoxicity test indicated that casein peptide-zinc chelates gastrointestinal digest were toxic to Caco-2 cells when the concentration exceeded 0.4 mg/mL. Finally, 15 and 13 dairy-derived peptides were identified from casein hydrolysate and casein peptide-zinc chelates, respectively, using mass spectrometry. These results will provide scientific evidence for the development and application of efficient casein peptide-zinc chelates.